Systems and methods for regulating heating assembly operation through pressure swing adsorption purge control

ABSTRACT

Pressure swing adsorption (PSA) assemblies with purge control systems, and hydrogen-generation assemblies and/or fuel cell systems containing the same. The PSA assemblies are operated according to a PSA cycle to produce a product hydrogen stream and a byproduct stream from a mixed gas stream. The byproduct stream may be delivered as a fuel stream to a heating assembly, which may heat the hydrogen-producing region that produces the mixed gas stream. The PSA assemblies may be adapted to regulate the flow of purge gas utilized therein, such as according to a predetermined, non-constant profile. In some embodiments, the flow of purge gas is regulated to maintain the flow rate and/or fuel value of the byproduct stream at or within a determined range of a threshold value, and/or to regulate the flow of purge gas to limit the concentration of carbon monoxide in a heated exhaust stream produced from the byproduct stream.

RELATED APPLICATIONS

This application is a continuing patent application that claims priorityto U.S. patent application Ser. No. 11/058,307, which was filed on Feb.14, 2005, issued on Jul. 15, 2008 as U.S. Pat. No. 7,399,342, and whichclaims priority to U.S. Provisional Patent Application Ser. No.60/638,451, which was filed on Dec. 22, 2004. The complete disclosuresof the above-identified patent and patent application are herebyincorporated by reference.

FIELD OF THE DISCLOSURE

The present disclosure is directed generally to hydrogen-generationassemblies that include pressure swing adsorption assemblies, and moreparticularly to systems and methods for regulating heating assemblyoperation in hydrogen-generation assemblies through control of the purgecycle of the pressure swing adsorption assemblies.

BACKGROUND OF THE DISCLOSURE

A hydrogen-generation assembly is an assembly that includes a fuelprocessing system that is adapted to convert one or more feedstocks intoa product stream containing hydrogen gas as a majority component. Theproduced hydrogen gas may be used in a variety of applications. One suchapplication is energy production, such as in electrochemical fuel cells.An electrochemical fuel cell is a device that converts a fuel and anoxidant to electricity, a reaction product, and heat. For example, fuelcells may convert hydrogen and oxygen into water and electricity. Insuch fuel cells, the hydrogen is the fuel, the oxygen is the oxidant,and the water is the reaction product. Fuel cells typically require highpurity hydrogen gas to prevent the fuel cells from being damaged duringuse. The product stream from the fuel processing system of ahydrogen-generation assembly may contain impurities, illustrativeexamples of which include one or more of carbon monoxide, carbondioxide, methane, unreacted feedstock, and water. Therefore, there is aneed in many conventional fuel cell systems to include suitablestructure for removing impurities from the impure hydrogen streamproduced in the fuel processing system.

A pressure swing adsorption (PSA) process is an example of a mechanismthat may be used to remove impurities from an impure hydrogen gas streamby selective adsorption of one or more of the impurities present in theimpure hydrogen stream. The adsorbed impurities can be subsequentlydesorbed and removed from the PSA assembly. PSA is a pressure-drivenseparation process that utilizes a plurality of adsorbent beds. The bedsare cycled through a series of steps, such as pressurization, separation(adsorption), depressurization (desorption), and purge steps toselectively remove impurities from the hydrogen gas and then desorb theimpurities.

Many hydrogen-generation assemblies include a heating assembly thatcombusts at least one fuel stream with air to produce a heated exhauststream for heating at least a portion of the hydrogen-generationassembly. The fuel streams may come from a variety of sources, includingthe PSA assembly. However, PSA assemblies are operated in PSA cyclesthat tend to produce exhaust, or byproduct, streams having varying andintermittent flows and/or varying fuel values. When used as a fuelstream for a heating assembly, this variation in flow rate and/or fuelvalue may produce inconsistent, often unpredictable, results in theheating assembly, such as periods of no fuel, periods of insufficientfuel, periods of too much fuel, periods in which the fuel streams havevariable fuel values, etc. As a result, it may be difficult for theheating assembly to maintain a selected component of thehydrogen-generation assembly at a desired temperature or within adesired, or selected, temperature range. Similarly, at times, the PSAassembly may not be producing sufficient, or any, exhaust stream tomaintain a flame or other ignition source of a heating assembly inoperation.

SUMMARY OF THE DISCLOSURE

The present disclosure is directed to PSA assemblies with purge controlsystems, as well as to hydrogen-generation assemblies and/or fuel cellsystems containing the same, and to methods of operating the same. ThePSA assemblies include at least one adsorbent bed, and typically aplurality of adsorbent beds, that include an adsorbent region includingadsorbent adapted to remove impurities from a mixed gas streamcontaining hydrogen gas as a majority component and other gases. Themixed gas stream may be produced by a hydrogen-producing region of afuel processing system, and the PSA assembly may produce a producthydrogen stream that is consumed by a fuel cell stack to provide a fuelcell system that produces electrical power. The PSA assembly produces abyproduct stream containing impurities removed from the mixed gas streamand a purge gas, which may be hydrogen gas, and a heating assembly maybe adapted to receive the byproduct stream as a fuel stream forgenerating a heated exhaust stream. The heated exhaust stream may beadapted to heat at least the hydrogen-producing region of a fuelprocessing system, such as to maintain the region at a suitabletemperature or within a suitable temperature range for producing themixed gas stream. The PSA assembly is adapted to regulate the flow ofpurge gas to the adsorbent beds during the purge steps of a PSA cycle.In some embodiments, the purge gas is selectively delivered according toa predetermined, non-constant profile. In some embodiments the profileincludes an initial flow rate that is less than the average flow rate ofpurge gas, and at least a subsequent flow rate that is greater than theaverage flow rate. In some embodiments, the flow of purge gas isregulated to maintain the flow rate and/or fuel value of the byproductstream at or within a determined range of a threshold value. In someembodiments, the flow of purge gas is regulated to limit theconcentration of carbon monoxide in a heated exhaust stream producedfrom the byproduct stream. In some embodiments, the PSA assemblyincludes a controller adapted to regulate the operation of at least thePSA assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an illustrative example of an energyproducing and consuming assembly that includes a hydrogen-generationassembly with an associated feedstock delivery system and a fuelprocessing system, as well as a fuel cell stack, and an energy-consumingdevice.

FIG. 2 is a schematic view of a hydrogen-producing assembly in the formof a steam reformer adapted to produce a reformate stream containinghydrogen gas and other gases from water and at least onecarbon-containing feedstock.

FIG. 3 is a schematic view of a fuel cell, such as may form part of afuel cell stack used with a hydrogen-generation assembly according tothe present disclosure.

FIG. 4 is a schematic view of a pressure swing adsorption assembly thatmay be used according to the present disclosure.

FIG. 5 is a schematic cross-sectional view of an adsorbent bed that maybe used with PSA assemblies according to the present disclosure.

FIG. 6 is a schematic cross-sectional view of another adsorbent bed thatmay be used with PSA assemblies according to the present disclosure.

FIG. 7 is a schematic cross-sectional view of another adsorbent bed thatmay be used with PSA assemblies according to the present disclosure.

FIG. 8 is a schematic cross-sectional view of the adsorbent bed of FIG.6 with a mass transfer zone being schematically indicated.

FIG. 9 is a schematic cross-sectional view of the adsorbent bed of FIG.8 with the mass transfer zone moved along the adsorbent region of thebed toward a distal, or product, end of the adsorbent region.

FIG. 10 is a schematic view of another example of a pressure swingadsorption assembly that may be used according to the presentdisclosure.

FIG. 11 is a schematic view of another example of a pressure swingadsorption assembly that may be used according to the presentdisclosure.

FIG. 12 is a graph depicting pressure within an adsorbent bed of a PSAassembly during the depressurization and purge steps of a PSA cycle.

FIG. 13 is a graph depicting the flow rate of purge gas to an adsorbentbed of a PSA assembly during the purge step of a PSA cycle according tothe present disclosure.

FIG. 14 is a graph depicting illustrative ramped flow rates of purge gasto an adsorbent bed of a PSA assembly during the purge step of a PSAcycle according to the present disclosure.

FIG. 15 is a graph depicting illustrative ramped flow rates of purge gasto an adsorbent bed of a PSA assembly during the purge step of a PSAcycle according to the present disclosure.

DETAILED DESCRIPTION AND BEST MODE OF THE DISCLOSURE

FIG. 1 illustrates schematically an example of an energy producing andconsuming assembly 56. The energy producing and consuming assembly 56includes an energy-producing system 22 and at least one energy-consumingdevice 52 adapted to exert an applied load on the energy-producingsystem 22. In the illustrated example, the energy-producing system 22includes a fuel cell stack 24 and a hydrogen-generation assembly 46.More than one of any of the illustrated components may be used withoutdeparting from the scope of the present disclosure. The energy-producingsystem may include additional components that are not specificallyillustrated in the schematic figures, such as air delivery systems, heatexchangers, sensors, controllers, flow-regulating devices, fuel and/orfeedstock delivery assemblies, heating assemblies, cooling assemblies,and the like. System 22 may also be referred to as a fuel cell system.

As discussed in more detail herein, hydrogen-generation assembliesand/or fuel cell systems according to the present disclosure include aseparation assembly that includes at least one pressure swing adsorption(PSA) assembly that is adapted to increase the purity of the hydrogengas that is produced in the hydrogen-generation assembly and/or consumedin the fuel cell stack. In a PSA process, gaseous impurities are removedfrom a stream containing hydrogen gas. PSA is based on the principlethat certain gases, under the proper conditions of temperature andpressure, will be adsorbed onto an adsorbent material more strongly thanother gases. These impurities may thereafter be desorbed and removed,such as in the form of a byproduct stream. The success of using PSA forhydrogen purification is due to the relatively strong adsorption ofcommon impurity gases (such as, but not limited to, CO, CO₂,hydrocarbons including CH₄, and N₂) on the adsorbent material. Hydrogenadsorbs only very weakly and so hydrogen passes through the adsorbentbed while the impurities are retained on the adsorbent material.

As discussed in more detail herein, a PSA process typically involvesrepeated, or cyclical, application of at least pressurization,separation (adsorption), depressurization (desorption), and purge steps,or processes, to selectively remove impurities from the hydrogen gas andthen desorb the impurities. Accordingly, the PSA process may bedescribed as being adapted to repeatedly enable a PSA cycle of steps, orstages, such as the above-described steps. The degree of separation isaffected by the pressure difference between the pressure of the mixedgas stream and the pressure of the byproduct stream Accordingly, thedesorption step will typically include reducing the pressure within theportion of the PSA assembly containing the adsorbed gases, andoptionally may even include drawing a vacuum (i.e., reducing thepressure to less than atmospheric or ambient pressure) on that portionof the assembly. Similarly, increasing the feed pressure of the mixedgas stream to the adsorbent regions of the PSA assembly may beneficiallyaffect the degree of separation during the adsorption step.

As illustrated schematically in FIG. 1, the hydrogen-generation assembly46 includes at least a fuel processing system 64 and a feedstockdelivery system 58, as well as the associated fluid conduitsinterconnecting various components of the system. As used herein, theterm “hydrogen-generation assembly” may be used to refer to the fuelprocessing system 64 and associated components of the energy-producingsystem, such as feedstock delivery systems 58, heating assemblies,separation regions or devices, air delivery systems, fuel deliverysystems, fluid conduits, heat exchangers, cooling assemblies, sensorassemblies, flow regulators, controllers, etc. All of these illustrativecomponents are not required to be included in any hydrogen-generationassembly or used with any fuel processing system according to thepresent disclosure. Similarly, other components may be included or usedas part of the hydrogen-generation assembly.

Regardless of its construction or components, the feedstock deliverysystem 58 is adapted to deliver to the fuel processing system 64 one ormore feedstocks via one or more streams, which may be referred togenerally as feedstock supply stream(s) 68. In the following discussion,reference may be made only to a single feedstock supply stream, but iswithin the scope of the present disclosure that two or more suchstreams, of the same or different composition, may be used. In someembodiments, air may be supplied to the fuel processing system 64 via ablower, fan, compressor or other suitable air delivery system, and/or awater stream may be delivered from a separate water source.

Fuel processing system 64 includes any suitable device(s) and/orstructure(s) that are configured to produce hydrogen gas from thefeedstock supply stream(s) 68. As schematically illustrated in FIG. 1,the fuel processing system 64 includes a hydrogen-producing region 70.Accordingly, fuel processing system 64 may be described as including ahydrogen-producing region 70 that produces a hydrogen-rich stream 74that includes hydrogen gas as a majority component from the feedstocksupply stream. While stream 74 contains hydrogen gas as its majoritycomponent, it also contains other gases, and as such may be referred toas a mixed gas stream that contains hydrogen gas and other gases.Illustrative, non-exclusive examples of these other gases, orimpurities, include one or more of such illustrative impurities ascarbon monoxide, carbon dioxide, water, methane, and unreactedfeedstock.

Illustrative examples of suitable mechanisms for producing hydrogen gasfrom feedstock supply stream 68 include steam reforming and autothermalreforming, in which reforming catalysts are used to produce hydrogen gasfrom a feedstock supply stream 68 containing water and at least onecarbon-containing feedstock. Other examples of suitable mechanisms forproducing hydrogen gas include pyrolysis and catalytic partial oxidationof a carbon-containing feedstock, in which case the feedstock supplystream 68 does not contain water. Still another suitable mechanism forproducing hydrogen gas is electrolysis, in which case the feedstock iswater. Illustrative examples of suitable carbon-containing feedstocksinclude at least one hydrocarbon or alcohol. Illustrative examples ofsuitable hydrocarbons include methane, propane, natural gas, diesel,kerosene, gasoline and the like. Illustrative examples of suitablealcohols include methanol, ethanol, and polyols, such as ethylene glycoland propylene glycol.

The hydrogen-generation assembly 46 may utilize more than a singlehydrogen-producing mechanism in the hydrogen-producing region 70 and mayinclude more than one hydrogen-producing region. Each of thesemechanisms is driven by, and results in, different thermodynamicbalances in the hydrogen-generation assembly 46. Accordingly, thehydrogen-generation assembly 46 may further include a temperaturemodulating assembly 71, such as a heating assembly and/or a coolingassembly. The temperature modulating assembly 71 may be configured aspart of the fuel processing system 64 or may be an external componentthat is in thermal and/or fluid communication with thehydrogen-producing region 70. The temperature modulating assembly 71 mayconsume a fuel stream, such as to generate heat. While not required inall embodiments of the present disclosure, the fuel stream may bedelivered from the feedstock delivery system. For example, and asindicated in dashed lines in FIG. 1, this fuel, or feedstock, may bereceived from the feedstock delivery system 58 via a fuel supply stream69. The fuel supply stream 69 may include combustible fuel or,alternatively, may include fluids to facilitate cooling. The temperaturemodulating assembly 71 may also receive some or all of its feedstockfrom other sources or supply systems, such as from additional storagetanks. It may also receive the air stream from any suitable source,including the environment within which the assembly is used. Blowers,fans and/or compressors may be used to provide the air stream, but thisis not required to all embodiments.

The temperature modulating assembly 71 may include one or more heatexchangers, burners, combustion systems, and other such devices forsupplying heat to regions of the fuel processing system and/or otherportions of assembly 56. Depending on the configuration of thehydrogen-generation assembly 46, the temperature modulating assembly 71may also, or alternatively, include heat exchangers, fans, blowers,cooling systems, and other such devices for cooling regions of the fuelprocessing system 64 or other portions of assembly 56. For example, whenthe fuel processing system 64 is configured with a hydrogen-producingregion 70 based on steam reforming or another endothermic reaction, thetemperature modulating assembly 71 may include systems for supplyingheat to maintain the temperature of the hydrogen-producing region 70 andthe other components in the proper range.

When the fuel processing system is configured with a hydrogen-producingregion 70 based on catalytic partial oxidation or another exothermicreaction, the temperature modulating assembly 71 may include systems forremoving heat, i.e., supplying cooling, to maintain the temperature ofthe fuel processing system in the proper range. As used herein, the term“heating assembly” is used to refer generally to temperature modulatingassemblies that are configured to supply heat or otherwise increase thetemperature of all or selected regions of the fuel processing system. Asused herein, the term “cooling assembly” is used to refer generally totemperature moderating assemblies that are configured to cool, or reducethe temperature of, all or selected regions of the fuel processingsystem.

In FIG. 2, an illustrative example of a hydrogen-generation assembly 46that includes fuel processing system 64 with a hydrogen-producing region70 that is adapted to produce mixed gas stream 74 by steam reforming oneor more feedstock supply streams 68 containing water 80 and at least onecarbon-containing feedstock 82. As illustrated, region 70 includes atleast one reforming catalyst bed 84 containing one or more suitablereforming catalysts 86. In the illustrative example, thehydrogen-producing region may be referred to as a reforming region, andthe mixed gas stream may be referred to as a reformate stream.

As also shown in FIGS. 1 and 2, the mixed gas stream is adapted to bedelivered to a separation region, or assembly, 72 that includes at leastone PSA assembly 73. PSA assembly 73 separates the mixed gas (orreformate) stream into product hydrogen stream 42 and at least onebyproduct stream 76 that contains at least a substantial portion of theimpurities, or other gases, present in mixed gas stream 74. Byproductstream 76 may contain no hydrogen gas, but it typically will containsome hydrogen gas. While not required, it is within the scope of thepresent disclosure that fuel processing system 64 may be adapted toproduce one or more byproduct streams containing sufficient amounts ofhydrogen (and/or other) gas(es) to be suitable for use as a fuel, orfeedstock, stream for a heating assembly for the fuel processing system.In some embodiments, the byproduct stream may have sufficient fuel value(i.e., hydrogen and/or other combustible gas content) to enable theheating assembly, when present, to maintain the hydrogen-producingregion at a desired operating temperature or within a selected range oftemperatures.

As illustrated in FIG. 2, the hydrogen-generation assembly includes atemperature modulating assembly in the form of a heating assembly 71that is adapted to produce a heated exhaust stream 88 that is adapted toheat at least the reforming region of the hydrogen-generation assembly.It is within the scope of the present disclosure that stream 88 may beused to heat other portions of the hydrogen-generation assembly and/orenergy-producing system 22.

As indicated in dashed lines in FIGS. 1 and 2, it is within the scope ofthe present disclosure that the byproduct stream from the PSA assemblymay form at least a portion of the fuel stream for the heating assembly.Also shown in FIG. 2 are air stream 90, which may be delivered from anysuitable air source, and fuel stream 92, which contains any suitablecombustible fuel suitable for being combusted with air in the heatingassembly. Fuel stream 92 may be used as the sole fuel stream for theheating assembly, but as discussed, it is also within the scope of thedisclosure that other combustible fuel streams may be used, such as thebyproduct stream from the PSA assembly, the anode exhaust stream from afuel cell stack, etc. When the byproduct or exhaust streams from othercomponents of system 22 have sufficient fuel value, fuel stream 92 maynot be used. When they do not have sufficient fuel value, are used forother purposes, or are not being generated, fuel stream 92 may be usedinstead or in combination.

Illustrative examples of suitable fuels include one or more of theabove-described carbon-containing feedstocks, although others may beused. As an illustrative example of temperatures that may be achievedand/or maintained in hydrogen-producing region 70 through the use ofheating assembly 71, steam reformers typically operate at temperaturesin the range of 200° C. and 900° C. Temperatures outside of this rangeare within the scope of the disclosure. When the carbon-containingfeedstock is methanol, the steam reforming reaction will typicallyoperate in a temperature range of approximately 200-500° C. Illustrativesubsets of this range include 350-450° C., 375-425° C., and 375-400° C.When the carbon-containing feedstock is a hydrocarbon, ethanol, or asimilar alcohol, a temperature range of approximately 400-900° C. willtypically be used for the steam reforming reaction. Illustrative subsetsof this range include 750-850° C., 725-825° C., 650-750° C., 700-800°C., 700-900° C., 500-800° C., 400-600° C., and 600-800° C.

It is within the scope of the present disclosure that the separationregion may be implemented within system 22 anywhere downstream from thehydrogen-producing region and upstream from the fuel cell stack. In theillustrative example shown schematically in FIG. 1, the separationregion is depicted as part of the hydrogen-generation assembly, but thisconstruction is not required. It is also within the scope of the presentdisclosure that the hydrogen-generation assembly may utilize a chemicalor physical separation process in addition to PSA assembly 73 to removeor reduce the concentration of one or more selected impurities from themixed gas stream. When separation assembly 72 utilizes a separationprocess in addition to PSA, the one or more additional processes may beperformed at any suitable location within system 22 and are not requiredto be implemented with the PSA assembly. An illustrative chemicalseparation process is the use of a methanation catalyst to selectivelyreduce the concentration of carbon monoxide present in stream 74. Otherillustrative chemical separation processes include partial oxidation ofcarbon monoxide to form carbon dioxide and water-gas shift reactions toproduce hydrogen gas and carbon dioxide from water and carbon monoxide.Illustrative physical separation processes include the use of a physicalmembrane or other barrier adapted to permit the hydrogen gas to flowtherethrough but adapted to prevent at least selected impurities frompassing therethrough. These membranes may be referred to as beinghydrogen-selective membranes. Illustrative examples of suitablemembranes are formed from palladium or a palladium alloy and aredisclosed in the references incorporated herein.

The hydrogen-generation assembly 46 preferably is adapted to produce atleast substantially pure hydrogen gas, and even more preferably, thehydrogen-generation assembly is adapted to produce pure hydrogen gas.For the purposes of the present disclosure, substantially pure hydrogengas is greater than 90% pure, preferably greater than 95% pure, morepreferably greater than 99% pure, and even more preferably greater than99.5% or even 99.9% pure. Illustrative, nonexclusive examples ofsuitable fuel processing systems are disclosed in U.S. Pat. Nos.6,221,117, 5,997,594, 5,861,137, and pending U.S. Patent ApplicationPublication Nos. 2001/0045061, 2003/0192251, and 2003/0223926. Thecomplete disclosures of the above-identified patents and patentapplications are hereby incorporated by reference for all purposes.

Hydrogen from the fuel processing system 64 may be delivered to one ormore of the storage device 62 and the fuel cell stack 24 via producthydrogen stream 42. Some or all of hydrogen stream 42 may additionally,or alternatively, be delivered, via a suitable conduit, for use inanother hydrogen-consuming process, burned for fuel or heat, or storedfor later use. With reference to FIG. 1, the hydrogen gas used as aproton source, or reactant, for fuel cell stack 24 may be delivered tothe stack from one or more of fuel processing system 64 and storagedevice 62. Fuel cell stack 24 includes at least one fuel cell 20, andtypically includes a plurality of fluidly and electricallyinterconnected fuel cells. When these cells are connected together inseries, the power output of the fuel cell stack is the sum of the poweroutputs of the individual cells. The cells in stack 24 may be connectedin series, parallel, or combinations of series and parallelconfigurations.

FIG. 3 illustrates schematically a fuel cell 20, one or more of whichmay be configured to form fuel cell stack 24. The fuel cell stacks ofthe present disclosure may utilize any suitable type of fuel cell, andpreferably fuel cells that receive hydrogen and oxygen as proton sourcesand oxidants. Illustrative examples of types of fuel cells includeproton exchange membrane (PEM) fuel cells, alkaline fuel cells, solidoxide fuel cells, molten carbonate fuel cells, phosphoric acid fuelcells, and the like. For the purpose of illustration, an exemplary fuelcell 20 in the form of a PEM fuel cell is schematically illustrated inFIG. 3.

Proton exchange membrane fuel cells typically utilize amembrane-electrode assembly 26 consisting of an ion exchange, orelectrolytic, membrane 29 located between an anode region 30 and acathode region 32. Each region 30 and 32 includes an electrode 34,namely an anode 36 and a cathode 38, respectively. Each region 30 and 32also includes a support 39, such as a supporting plate 40. Support 39may form a portion of the bipolar plate assemblies that are discussed inmore detail herein. The supporting plates 40 of fuel cells 20 carry therelative voltage potentials produced by the fuel cells.

In operation, hydrogen gas from product stream 42 is delivered to theanode region, and oxidant 44 is delivered to the cathode region. Atypical, but not exclusive, oxidant is oxygen. As used herein, hydrogenrefers to hydrogen gas and oxygen refers to oxygen gas. The followingdiscussion will refer to hydrogen as the proton source, or fuel, for thefuel cell (stack), and oxygen as the oxidant, although it is within thescope of the present disclosure that other fuels and/or oxidants may beused. Hydrogen and oxygen 44 may be delivered to the respective regionsof the fuel cell via any suitable mechanism from respective sources 47and 48. Illustrative examples of suitable sources 48 of oxygen 44include a pressurized tank of oxygen or air, or a fan, compressor,blower or other device for directing air to the cathode region.

Hydrogen and oxygen typically combine with one another via anoxidation-reduction reaction. Although membrane 29 restricts the passageof a hydrogen molecule, it will permit a hydrogen ion (proton) to passthrough it, largely due to the ionic conductivity of the membrane. Thefree energy of the oxidation-reduction reaction drives the proton fromthe hydrogen gas through the ion exchange membrane. As membrane 29 alsotends not to be electrically conductive, an external circuit 50 is thelowest energy path for the remaining electron, and is schematicallyillustrated in FIG. 3. In cathode region 32, electrons from the externalcircuit and protons from the membrane combine with oxygen to producewater and heat.

Also shown in FIG. 3 are an anode purge, or exhaust, stream 54, whichmay contain hydrogen gas, and a cathode air exhaust stream 55, which istypically at least partially, if not substantially, depleted of oxygen.Fuel cell stack 24 may include a common hydrogen (or other reactant)feed, air intake, and stack purge and exhaust streams, and accordinglywill include suitable fluid conduits to deliver the associated streamsto, and collect the streams from, the individual fuel cells. Similarly,any suitable mechanism may be used for selectively purging the regions.

In practice, a fuel cell stack 24 will typically contain a plurality offuel cells with bipolar plate assemblies separating adjacentmembrane-electrode assemblies. The bipolar plate assemblies essentiallypermit the free electron to pass from the anode region of a first cellto the cathode region of the adjacent cell via the bipolar plateassembly, thereby establishing an electrical potential through the stackthat may be used to satisfy an applied load. This net flow of electronsproduces an electric current that may be used to satisfy an appliedload, such as from at least one of an energy-consuming device 52 and theenergy-producing system 22.

For a constant output voltage, such as 12 volts or 24 volts, the outputpower may be determined by measuring the output current. The electricaloutput may be used to satisfy an applied load, such as fromenergy-consuming device 52. FIG. 1 schematically depicts thatenergy-producing system 22 may include at least one energy-storagedevice 78. Device 78, when included, may be adapted to store at least aportion of the electrical output, or power, 79 from the fuel cell stack24. An illustrative example of a suitable energy-storage device 78 is abattery, but others may be used. Energy-storage device 78 mayadditionally or alternatively be used to power the energy-producingsystem 22 during start-up of the system.

The at least one energy-consuming device 52 may be electrically coupledto the energy-producing system 22, such as to the fuel cell stack 24and/or one or more energy-storage devices 78 associated with the stack.Device 52 applies a load to the energy-producing system 22 and draws anelectric current from the system to satisfy the load. This load may bereferred to as an applied load, and may include thermal and/orelectrical load(s). It is within the scope of the present disclosurethat the applied load may be satisfied by the fuel cell stack, theenergy-storage device, or both the fuel cell stack and theenergy-storage device. Illustrative examples of devices 52 include motorvehicles, recreational vehicles, boats and other sea craft, and anycombination of one or more residences, commercial offices or buildings,neighborhoods, tools, lights and lighting assemblies, appliances,computers, industrial equipment, signaling and communications equipment,radios, electrically powered components on boats, recreational vehiclesor other vehicles, battery chargers and even the balance-of-plantelectrical requirements for the energy-producing system 22 of which fuelcell stack 24 forms a part. As indicated in dashed lines at 77 in FIG.1, the energy-producing system may, but is not required to, include atleast one power management module 77. Power management module 77includes any suitable structure for conditioning or otherwise regulatingthe electricity produced by the energy-producing system, such as fordelivery to energy-consuming device 52. Module 77 may include suchillustrative structure as buck or boost converters, inverters, powerfilters, and the like.

In FIG. 4 an illustrative example of a PSA assembly 73 is shown. Asshown, assembly 73 includes a plurality of adsorbent beds 100 that arefluidly connected via distribution assemblies 102 and 104. Beds 100 mayadditionally or alternatively be referred to as adsorbent chambers oradsorption regions. The distribution assemblies have been schematicallyillustrated in FIG. 4 and may include any suitable structure forselectively establishing and restricting fluid flow between the bedsand/or the input and output streams of assembly 73. As shown, the inputand output streams include at least mixed gas stream 74, producthydrogen stream 42, and byproduct stream 76. Illustrative examples ofsuitable structures include one or more of manifolds, such asdistribution and collection manifolds that are respectively adapted todistribute fluid to and collect fluid from the beds, and valves, such ascheck valves, solenoid valves, purge valves, and the like. In theillustrative example, three beds 100 are shown, but it is within thescope of the present disclosure that the number of beds may vary, suchas to include more or less beds than shown in FIG. 4. Typically,assembly 73 will include at least two beds, and often will includethree, four, or more beds. While not required, assembly 73 is preferablyadapted to provide a continuous flow of product hydrogen stream, with atleast one of the plurality of beds exhausting this stream when theassembly is in use and receiving a continuous flow of mixed gas stream74.

In the illustrative example, distribution assembly 102 is adapted toselectively deliver mixed gas stream 74 to the plurality of beds and tocollect and exhaust byproduct stream 76, and distribution assembly 104is adapted to collect the purified hydrogen gas that passes through thebeds and which forms product hydrogen stream 42, and in some embodimentsto deliver a portion of the purified hydrogen gas to the beds for use asa purge stream. The distribution assemblies may be configured for fixedor rotary positioning relative to the beds. Furthermore, thedistribution assemblies may include any suitable type and number ofstructures and devices to selectively distribute, regulate, meter,prevent and/or collect flows of the corresponding gas streams. Asillustrative, non-exclusive examples, distribution assembly 102 mayinclude mixed gas and exhaust manifolds, or manifold assemblies, anddistribution assembly 104 may include product and purge manifolds, ormanifold assemblies. In practice, PSA assemblies that utilizedistribution assemblies that rotate relative to the beds may be referredto as rotary pressure swing adsorption assemblies, and PSA assemblies inwhich the manifolds and beds are not adapted to rotate relative to eachother to selectively establish and restrict fluid connections may bereferred to as fixed bed, or discrete bed, pressure swing adsorptionassemblies. Both constructions are within the scope of the presentdisclosure.

Gas purification by pressure swing adsorption involves sequentialpressure cycling and flow reversal of gas streams relative to theadsorbent beds. In the context of purifying a mixed gas stream comprisedsubstantially of hydrogen gas, the mixed gas stream is delivered underrelatively high pressure to one end of the adsorbent beds and therebyexposed to the adsorbent(s) contained in the adsorbent region thereof.Illustrative examples of delivery pressures for mixed gas stream 74include pressures in the range of 40-200 psi, such as pressures in therange of 50-150 psi, 50-100 psi, 100-150 psi, 70-100 psi, etc., althoughpressures outside of this range are within the scope of the presentdisclosure. As the mixed gas stream flows through the adsorbent region,carbon monoxide, carbon dioxide, water and/or other ones of theimpurities, or other gases, are adsorbed, and thereby at leasttemporarily retained, on the adsorbent. This is because these gases aremore readily adsorbed on the selected adsorbents used in the PSAassembly. The remaining portion of the mixed gas stream, which now mayperhaps more accurately be referred to as a purified hydrogen stream,passes through the bed and is exhausted from the other end of the bed.In this context, hydrogen gas may be described as being the less readilyadsorbed component, while carbon monoxide, carbon dioxide, etc. may bedescribed as the more readily adsorbed components of the mixed gasstream. The pressure of the product hydrogen stream is typically reducedprior to utilization of the gas by the fuel cell stack.

To remove the adsorbed gases, the flow of the mixed gas stream isstopped, the pressure in the bed is reduced, and the now desorbed gasesare exhausted from the bed. The desorption step often includesselectively decreasing the pressure within the adsorbent region throughthe withdrawal of gas, typically in a countercurrent direction relativeto the feed direction. This desorption step may also be referred to as adepressurization, or blowdown, step. This step often includes or isperformed in conjunction with the use of a purge gas stream, which istypically delivered in a countercurrent flow direction to the directionat which the mixed gas stream flows through the adsorbent region. Anillustrative example of a suitable purge gas stream is a portion of theproduct hydrogen stream, as this stream is comprised of hydrogen gas,which is less readily adsorbed than the adsorbed gases. Other gases maybe used in the purge gas stream, although these gases preferably areless readily adsorbed than the adsorbed gases, and even more preferablyare not adsorbed, or are only weakly adsorbed, on the adsorbent(s) beingused.

As discussed, this desorption step may include drawing an at leastpartial vacuum on the bed, but this is not required. While not required,it is often desirable to utilize one or more equalization steps, inwhich two or more beds are fluidly interconnected to permit the beds toequalize the relative pressures therebetween. For example, one or moreequalization steps may precede the desorption and pressurization steps.Prior to the desorption step, equalization is used to reduce thepressure in the bed and to recover some of the purified hydrogen gascontained in the bed, while prior to the (re)pressurization step,equalization is used to increase the pressure within the bed.Equalization may be accomplished using cocurrent and/or countercurrentflow of gas. After the desorption and/or purge step(s) of the desorbedgases is completed, the bed is again pressurized and ready to againreceive and remove impurities from the portion of the mixed gas streamdelivered thereto.

For example, when a bed is ready to be regenerated, it is typically at arelatively high pressure and contains a quantity of hydrogen gas. Whilethis gas (and pressure) may be removed simply by venting the bed, otherbeds in the assembly will need to be pressurized prior to being used topurify the portion of the mixed gas stream delivered thereto.Furthermore, the hydrogen gas in the bed to be regenerated preferably isrecovered so as to not negatively impact the efficiency of the PSAassembly. Therefore, interconnecting these beds in fluid communicationwith each other permits the pressure and hydrogen gas in the bed to beregenerated to be reduced while also increasing the pressure andhydrogen gas in a bed that will be used to purify impure hydrogen gas(i.e., mixed gas stream 74) that is delivered thereto. In addition to,or in place of, one or more equalization steps, a bed that will be usedto purify the mixed gas stream may be pressurized prior to the deliveryof the mixed gas stream to the bed. For example, some of the purifiedhydrogen gas may be delivered to the bed to pressurize the bed. While itis within the scope of the present disclosure to deliver thispressurization gas to either end of the bed, in some embodiments it maybe desirable to deliver the pressurization gas to the opposite end ofthe bed than the end to which the mixed gas stream is delivered.

The above discussion of the general operation of a PSA assembly has beensomewhat simplified. Illustrative examples of pressure swing adsorptionassemblies, including components thereof and methods of operating thesame, are disclosed in U.S. Pat. Nos. 3,564,816, 3,986,849, 5,441,559,6,692,545, and 6,497,856, the complete disclosures of which are herebyincorporated by reference for all purposes.

In FIG. 5, an illustrative example of an adsorbent bed 100 isschematically illustrated. As shown, the bed defines an internalcompartment 110 that contains at least one adsorbent 112, with eachadsorbent being adapted to adsorb one or more of the components of themixed gas stream. It is within the scope of the present disclosure thatmore than one adsorbent may be used. For example, a bed may include morethan one adsorbent adapted to adsorb a particular component of the mixedgas stream, such as to adsorb carbon monoxide, and/or two or moreadsorbents that are each adapted to adsorb a different component of themixed gas stream. Similarly, an adsorbent may be adapted to adsorb twoor more components of the mixed gas stream. Illustrative examples ofsuitable adsorbents include activated carbon, alumina and zeoliteadsorbents. An additional example of an adsorbent that may be presentwithin the adsorbent region of the beds is a desiccant that is adaptedto adsorb water present in the mixed gas stream. Illustrative desiccantsinclude silica and alumina gels. When two or more adsorbents areutilized; they may be sequentially positioned (in a continuous ordiscontinuous relationship) within the bed or may be mixed together. Itshould be understood that the type, number, amount and form of adsorbentin a particular PSA assembly may vary, such as according to one or moreof the following factors: the operating conditions expected in the PSAassembly, the size of the adsorbent bed, the composition and/orproperties of the mixed gas stream, the desired application for theproduct hydrogen stream produced by the PSA assembly, the operatingenvironment in which the PSA assembly will be used, user preferences,etc.

When the PSA assembly includes a desiccant or other water-removalcomposition or device, it may be positioned to remove water from themixed gas stream prior to adsorption of other impurities from the mixedgas stream. One reason for this is that water may negatively affect theability of some adsorbents to adsorb other components of the mixed gasstream, such as carbon monoxide. An illustrative example of awater-removal device is a condenser, but others may be used between thehydrogen-producing region and adsorbent region, as schematicallyillustrated in dashed lines at 122 in FIG. 1. For example, at least oneheat exchanger, condenser or other suitable water-removal device may beused to cool the mixed gas stream prior to delivery of the stream to thePSA assembly. This cooling may condense some of the water present in themixed gas stream. Continuing this example, and to provide a morespecific illustration, mixed gas streams produced by steam reformerstend to contain at least 10%, and often at least 15% or more water whenexhausted from the hydrogen-producing (i.e., the reforming) region ofthe fuel processing system. These streams also tend to be fairly hot,such as having a temperature of at least 300° C. (in the case of manymixed gas streams produced from methanol or similar carbon-containingfeedstocks), and at least 600-800° C. (in the case of many mixed gasstreams produced from natural gas, propane or similar carbon-containingfeedstocks). When cooled prior to delivery to the PSA assembly, such asto an illustrative temperature in the range of 25-100° C. or even 40-80°C., most of this water will condense. The mixed gas stream may still besaturated with water, but the water content will tend to be less than 5wt %.

The adsorbent(s) may be present in the bed in any suitable form,illustrative examples of which include particulate form, bead form,porous discs or blocks, coated structures, laminated sheets, fabrics,and the like. When positioned for use in the beds, the adsorbents shouldprovide sufficient porosity and/or gas flow paths for the non-adsorbedportion of the mixed gas stream to flow through the bed withoutsignificant pressure drop through the bed. As used herein, the portionof a bed that contains adsorbent will be referred to as the adsorbentregion of the bed. In FIG. 5, an adsorbent region is indicated generallyat 114. Beds 100 also may (but are not required to) include partitions,supports, screens and other suitable structure for retaining theadsorbent and other components of the bed within the compartment, inselected positions relative to each other, in a desired degree ofcompression, etc. These devices are generally referred to as supportsand are generally indicated in FIG. 5 at 116. Therefore, it is withinthe scope of the present disclosure that the adsorbent region maycorrespond to the entire internal compartment of the bed, or only asubset thereof. Similarly, the adsorbent region may be comprised of acontinuous region or two or more spaced apart regions without departingfrom the scope of the present disclosure.

In the illustrated example shown in FIG. 5, bed 100 includes at leastone port 118 associated with each end region of the bed. As indicated indashed lines, it is within the scope of the present disclosure thateither or both ends of the bed may include more than one port.Similarly, it is within the scope of the disclosure that the ports mayextend laterally from the beds or otherwise have a different geometrythan the schematic examples shown in FIG. 5. Regardless of theconfiguration and/or number of ports, the ports are collectively adaptedto deliver fluid for passage through the adsorbent region of the bed andto collect fluid that passes through the adsorbent region. As discussed,the ports may selectively, such as depending upon the particularimplementation of the PSA assembly and/or stage in the PSA cycle, beused as an input port or an output port. For the purpose of providing agraphical example, FIG. 6 illustrates a bed 100 in which the adsorbentregion extends along the entire length of the bed, i.e., between theopposed ports or other end regions of the bed, In FIG. 7, bed 100includes an adsorbent region 114 that includes discontinuous subregions120.

During use of an adsorbent bed, such as bed 100, to adsorb impuritygases (namely the gases with greater affinity for being adsorbed by theadsorbent), a mass-transfer zone will be defined in the adsorbentregion. More particularly, adsorbents have a certain adsorptioncapacity, which is defined at least in part by the composition of themixed gas stream, the flow rate of the mixed gas stream, the operatingtemperature and/or pressure at which the adsorbent is exposed to themixed gas stream, any adsorbed gases that have not been previouslydesorbed from the adsorbent, etc. As the mixed gas stream is deliveredto the adsorbent region of a bed, the adsorbent at the end portion ofthe adsorbent region proximate the mixed gas delivery port will removeimpurities from the mixed gas stream. Generally, these impurities willbe adsorbed within a subset of the adsorbent region, and the remainingportion of the adsorbent region will have only minimal, if any, adsorbedimpurity gases. This is somewhat schematically illustrated in FIG. 8, inwhich adsorbent region 114 is shown including a mass transfer zone, orregion, 130.

As the adsorbent in the initial mass transfer zone continues to adsorbimpurities, it will near or even reach its capacity for adsorbing theseimpurities. As this occurs, the mass transfer zone will move toward theoppose end of the adsorbent region. More particularly, as the flow ofimpurity gases exceeds the capacity of a particular portion of theadsorbent region (i.e., a particular mass transfer zone) to adsorb thesegases, the gases will flow beyond that region and into the adjoiningportion of the adsorbent region, where they will be adsorbed by theadsorbent in that portion, effectively expanding and/or moving the masstransfer zone generally toward the opposite end of the bed.

This description is somewhat simplified in that the mass transfer zoneoften does not define uniform beginning and ending boundaries along theadsorbent region, especially when the mixed gas stream contains morethan one gas that is adsorbed by the adsorbent. Similarly, these gasesmay have different affinities for being adsorbed and therefore may evencompete with each other for adsorbent sites. However, a substantialportion (such as at least 70% or more) of the adsorption will tend tooccur in a relatively localized portion of the adsorbent region, withthis portion, or zone, tending to migrate from the feed end to theproduct end of the adsorbent region during use of the bed. This isschematically illustrated in FIG. 9, in which mass transfer zone 130 isshown moved toward port 118′ relative to its position in FIG. 8.Accordingly, the adsorbent 112′ in portion 114′ of the adsorbent regionwill have a substantially reduced capacity, if any, to adsorb additionalimpurities. Described in other terms, adsorbent 112′ may be described asbeing substantially, if not completely, saturated with adsorbed gases.In FIGS. 8 and 9, the feed and product ends of the adsorbent region aregenerally indicated at 124 and 126 and generally refer to the portionsof the adsorbent region that are proximate, or closest to, the mixed gasdelivery port and the product port of the bed.

During use of the PSA assembly, the mass transfer zone will tend tomigrate toward and away from ends 124 and 126 of the adsorbent region.More specifically, and as discussed, PSA is a cyclic process thatinvolves repeated changes in pressure and flow direction. The followingdiscussion will describe the PSA cycle with reference to how steps inthe cycle tend to affect the mass transfer zone (and/or the distributionof adsorbed gases through the adsorbent region). It should be understoodthat the size, or length, of the mass transfer zone will tend to varyduring use of the PSA assembly, and therefore tends not to be of a fixeddimension.

At the beginning of a PSA cycle, the bed is pressurized and the mixedgas stream flows under pressure through the adsorbent region. Duringthis adsorption step, impurities (i.e., the other gases) are adsorbed bythe adsorbent(s) in the adsorbent region. As these impurities areadsorbed, the mass transfer zone tends to move toward the distal, orproduct, end of the adsorbent region as initial portions of theadsorbent region become more and more saturated with adsorbed gas. Whenthe adsorption step is completed, the flow of mixed gas stream 74 to theadsorbent bed and the flow of purified hydrogen gas (at least a portionof which will form product hydrogen stream 42) are stopped. While notrequired, the bed may then undergo one or more equalization steps inwhich the bed is fluidly interconnected with one or more other beds inthe PSA assembly to decrease the pressure and hydrogen gas present inthe bed and to charge the receiving bed(s) with pressure and hydrogengas. Gas may be withdrawn from the pressurized bed from either, or bothof, the feed or the product ports. Drawing the gas from the product portwill tend to provide hydrogen gas of greater purity than gas drawn fromthe feed port. However, the decrease in pressure resulting from thisstep will tend to draw impurities in the direction at which the gas isremoved from the adsorbent bed. Accordingly, the mass transfer zone maybe described as being moved toward the end of the adsorbent bed closestto the port from which the gas is removed from the bed. Expressed indifferent terms, when the bed is again used to adsorb impurities fromthe mixed gas stream, the portion of the adsorbent region in which themajority of the impurities are adsorbed at a given time, i.e., the masstransfer zone, will tend to be moved toward the feed or product end ofthe adsorbent region depending upon the direction at which theequalization gas is withdrawn from the bed.

The bed is then depressurized, with this step typically drawing gas fromthe feed port because the gas stream will tend to have a higherconcentration of the other gases, which are desorbed from the adsorbentas the pressure in the bed is decreased. This exhaust stream may bereferred to as a byproduct, or impurity stream, 76 and may be used for avariety of applications, including as a fuel stream for a burner orother heating assembly that combusts a fuel stream to produce a heatedexhaust stream, As discussed, hydrogen-generation assembly 46 mayinclude a heating assembly 71 that is adapted to produce a heatedexhaust stream to heat at least the hydrogen-producing region 70 of thefuel processing system. According to Henry's Law, the amount of adsorbedgases that are desorbed from the adsorbent is related to the partialpressure of the adsorbed gas present in the adsorbent bed. Therefore,the depressurization step may include, be followed by, or at leastpartially overlap in time, with a purge step, in which gas, typically atlow pressure, is introduced into the adsorbent bed. This gas flowsthrough the adsorbent region and draws the desorbed gases away from theadsorbent region, with this removal of the desorbed gases resulting infurther desorption of gas from the adsorbent. As discussed, a suitablepurge gas is purified hydrogen gas, such as previously produced by thePSA assembly. Typically, the purge stream flows from the product end tothe feed end of the adsorbent region to urge the impurities (and thusreposition the mass transfer zone) toward the feed end of the adsorbentregion. It is within the scope of the disclosure that the purge gasstream may form a portion of the byproduct stream, may be used as acombustible fuel stream (such as for heating assembly 71), and/or may beotherwise utilized in the PSA or other processes.

The illustrative example of a PSA cycle is now completed, and a newcycle is typically begun. For example, the purged adsorbent bed is thenrepressurized, such as by being a receiving bed for another adsorbentbed undergoing equalization, and optionally may be further pressurizedby purified hydrogen gas delivered thereto. By utilizing a plurality ofadsorbent beds, typically three or more, the PSA assembly may be adaptedto receive a continuous flow of mixed gas stream 74 and to produce acontinuous flow of purified hydrogen gas (i.e., a continuous flow ofproduct hydrogen stream 42). While not required, the time for theadsorption step, or stage, often represents one-third to two-thirds ofthe PSA cycle, such as representing approximately half of the time for aPSA cycle.

It is important to stop the adsorption step before the mass transferzone reaches the distal end (relative to the direction at which themixed gas stream is delivered to the adsorbent region) of the adsorbentregion. In other words, the flow of mixed gas stream 74 and the removalof product hydrogen stream 42 preferably should be stopped before theother gases that are desired to be removed from the hydrogen gas areexhausted from the bed with the hydrogen gas because the adsorbent issaturated with adsorbed gases and therefore can no longer effectivelyprevent these impurity gases from being exhausted in what desirably is apurified hydrogen stream. This contamination of the product hydrogenstream with impurity gases that desirably are removed by the PSAassembly may be referred to as breakthrough, in that the impuritiesgases “break through” the adsorbent region of the bed. Conventionally,carbon monoxide detectors have been used to determine when the masstransfer zone is nearing or has reached the distal end of the adsorbentregion and thereby is, or will, be present in the product hydrogenstream. Carbon monoxide detectors are used more commonly than detectorsfor other ones of the other gases present in the mixed gas streambecause carbon monoxide can damage many fuel cells when present in evena few parts per million (ppm). While effective, and within the scope ofthe present disclosure, this detection mechanism requires the use ofcarbon monoxide detectors and related detection equipment, which tendsto be expensive and increase the complexity of the PSA assembly.

As introduced in connection with FIG. 4, PSA assembly 73 includesdistribution assemblies 102 and 104 that selectively deliver and/orcollect mixed gas stream 74, product hydrogen stream 42, and byproductstream 76 to and from the plurality of adsorbent beds 100. As discussed,product hydrogen stream 42 is formed from the purified hydrogen gasstreams produced in the adsorbent regions of the adsorbent beds. It iswithin the scope of the present disclosure that some of this gas may beused as a purge gas stream that is selectively delivered (such as via anappropriate distribution manifold) to the adsorbent beds during thepurge and/or blowdown steps to promote the desorption and removal of theadsorbed gases for the adsorbent. The desorbed gases, as well as thepurge gas streams that are withdrawn from the adsorbent beds with thedesorbed gases collectively may form byproduct stream 76, which asdiscussed, may be used as a fuel stream for heating assembly 71 or otherdevice that is adapted to receive a combustible fuel stream.

FIGS. 10 and 11 provide a somewhat less schematic example of PSAassemblies 73 that include a plurality of adsorbent beds 100. Similar tothe illustrative example shown in FIG. 4, three adsorbent beds are shownin FIG. 10, but it is within the scope of the present disclosure thatmore or less beds may be utilized, as graphically depicted in FIG. 11,in which four beds are shown, although more than four beds may beutilized without departing from the scope of the present disclosure.Similarly, more than one PSA assembly may be used in connection with thesame hydrogen-generation assembly and/or fuel cell system. As shown inFIGS. 10 and 11, PSA assembly 73 includes a distribution assembly 102that includes a mixed gas manifold 140 and an exhaust manifold 142.Mixed gas manifold 140 is adapted to selectively distribute the mixedgas stream to the feed ends 144 of the adsorbent beds, as indicated at74′. Exhaust manifold 142 is adapted to collect gas that is exhaustedfrom the feed ends of the adsorbent beds, namely, the desorbed othergases, purge gas, and other gas that is not harvested to form producthydrogen stream 42. These exhausted streams are indicated at 76′ inFIGS. 10 and 11 and collectively form byproduct stream 76.

FIGS. 10 and 11 also schematically depict byproduct stream 76 beingdelivered to heating assembly 71 to be combusted with air, such as fromair stream 90, to produce heated exhaust stream 88. As also shown inFIGS. 10 and 11, it is within the scope of the present disclosure thatheating assembly 71 may, but is not required to, be adapted to receive afuel stream 92 in addition to byproduct stream 76. In some embodiments,stream 92 may also be referred to as a supplemental fuel stream. Anysuitable combustible fuel may be used in stream 92. Illustrativeexamples of suitable fuels for stream 92 include hydrogen gas, such ashydrogen gas produced by hydrogen-generation assembly 46, and/or any ofthe above-discussed carbon-containing feedstocks, including withoutlimitation propane, natural gas, methane, and methanol.

As discussed in connection with FIG. 2, when PSA assembly 73 and heatingassembly 71 are used in connection with a fuel processing system 64 thatincludes a hydrogen-producing region 70 that operates at elevatedtemperatures, the heating assembly may be adapted to heat at leastregion 70 with exhaust stream 88. For example, stream 88 may heat region70 to a suitable temperature and/or to within a suitable temperaturerange, for producing hydrogen gas from one or more feed streams. As alsodiscussed, steam and autothermal reforming reactions are illustrativeexamples of endothermic processes that may be used to produce mixed gasstream 74 from water and a carbon-containing feedstock, although otherprocesses and/or feed stream components may additionally oralternatively be used to produce mixed gas stream 74. It is also withinthe scope of the present disclosure that the exhaust stream may beadapted to provide primary heating to heat to a component of ahydrogen-production assembly, fuel cell system, or other implementationof assemblies 71 and 73.

In the illustrative embodiments shown in FIGS. 10 and 1 distributionassembly 104 includes a product manifold 150 and a purge manifold 152.Product manifold 150 is adapted to collect the streams of purifiedhydrogen gas that are withdrawn from the product ends 154 of theadsorbent beds and from which product hydrogen stream 42 is formed.These streams of purified hydrogen gas are indicated in FIGS. 10 and 11at 42′. Purge manifold 152 is adapted to selectively deliver a purgegas, such as a portion of the purified hydrogen gas, to the adsorbedbeds, such as to promote desorption of the adsorbed impurity gases andthereby regenerate the adsorbent contained therein. The purge gasstreams are indicated at 156′ and may be collectively referred to as apurge gas stream 156. As indicated at 158, it is within the scope of thepresent disclosure that the product and purge manifolds may be in fluidcommunication with each other to selectively divert at least a portionof the purified hydrogen gas (or product hydrogen stream) to be used aspurge stream 156. It is also within the scope of the present disclosurethat one or more other gases from one or more other sources mayadditionally or alternatively form at least a portion of purge stream156.

Although not required, FIGS. 10 and 11 illustrate at 160 that in someembodiments it may be desirable to fluidly connect the product manifoldand/or fluid conduits for the product hydrogen stream with the fluidconduits for the byproduct stream. Such a fluid connection may be usedto selectively divert at least a portion of the purified (orintended-to-be-purified) hydrogen gas to the heating assembly instead ofto the destination to which product hydrogen stream 42 otherwise isdelivered. As discussed, examples of suitable destinations includehydrogen storage devices, fuel cell stacks and hydrogen-consumingdevices. Illustrative examples of situations in which the diversion ofthe product hydrogen stream to the heating assembly include if thedestination is already receiving its maximum capacity of hydrogen gas,is out of service or otherwise unable to receive any or additionalhydrogen gas, if an unacceptable concentration of one or more impuritiesare detected in the hydrogen gas, if it is necessary to shutdown thehydrogen-generation assembly and/or fuel cell system, if a portion ofthe product hydrogen stream is needed as a fuel stream for the heatingassembly, etc.

In an implemented embodiment of PSA assembly 73, any suitable number,structure and construction of manifolds and fluid conduits for the fluidstreams discussed herein may be utilized. Similarly, any suitable numberand type of valves or other flow-regulating devices 170 and/or sensorsor other property detectors 172 may be utilized, illustrative,non-exclusive examples of which are shown in FIGS. 10 and/or 11. Forexample, check valves 174, proportioning or other solenoid valves 176,pressure relief valves 178, variable orifice valves 180, and fixedorifices 182 are shown to illustrate non-exclusive examples offlow-regulating devices 170. Similarly, flow meters 190, pressuresensors 192, temperature sensors 194, and composition detectors 196 areshown to illustrate non-exclusive examples of property detectors 172. Anillustrative example of a composition detector is a carbon monoxidedetector 198, such as to detect the concentration, if any, of carbonmonoxide in the purified hydrogen gas streams 42′ and/or producthydrogen stream 42.

While not required, it is within the scope of the present disclosurethat the PSA assembly may include, be associated with, and/or be incommunication with a controller that is adapted to control the operationof at least portions of the PSA assembly and/or an associatedhydrogen-generation assembly and/or fuel cell system. A controller isschematically illustrated in FIGS. 2 and 10-11 and generally indicatedat 200. Controller 200 may communicate with at least the flow-regulatingdevices and/or property detectors 172 via any suitable wired and/orwireless communication linkage, as schematically illustrated at 202.This communication may include one- or two-way communication and mayinclude such communication signals as inputs and/or outputscorresponding to measured or computed values, command signals, statusinformation, user inputs, values to be stored, threshold values, etc. Asillustrative, non-exclusive examples, controller 200 may include one ormore analog or digital circuits, logic units or processors for operatingprograms stored as software in memory, one or more discrete units incommunication with each other, etc. Controller 200 may also regulate orcontrol other portions of the hydrogen-generation assembly or fuel cellsystem and/or may be in communication with other controllers adapted tocontrol the operation of the hydrogen-generation assembly and/or fuelcell system. Controller 200 is illustrated in FIGS. 10 and 11 as beingimplemented as a discrete unit. It may also be implemented as separatecomponents or controllers. Such separate controllers, then, cancommunicate with each other and/or with other controllers present insystem 22 and/or assembly 46 via any suitable communication linkages.

In FIGS. 10 and 11, a plurality of temperature sensors 194 are shownassociated with one of the illustrated adsorbent beds. It is within thescope of the present disclosure that each or none of the beds mayinclude one or more temperature sensors adapted to detect one or moretemperatures associated with the adsorbent bed, the adsorbent in thebed, the adsorbent region of the bed, the gas flowing through the bed,etc. Although not required, PSA assemblies 73 according to the presentdisclosure may include a temperature-based breakthrough detectionsystem, such as disclosed in U.S. Provisional Patent Application Ser.No. 60/638,086, which was filed on Dec. 20, 2004, is entitled“Temperature-Based Breakthrough Detection and Pressure Swing AdsorptionSystems and Fuel Processing Systems Including the Same,” and thecomplete disclosure of which is hereby incorporated by reference for allpurposes.

In FIG. 12, an illustrative (non-exclusive) graph of the pressure withinan adsorbent bed during the blowdown, or depressurization, and purgesteps of a PSA cycle is shown. At 210, the pressure is indicated priorto the depressurization step, such as after the adsorption step, orstage, has been completed, and perhaps more typically, after one or moreequalization steps have been completed. The initiation of the flow ofgas from the feed end of the bed during the depressurization, ordesorption, step is indicated at 210, and as somewhat schematicallyindicated, the pressure drops relatively quickly. The rate of decreasemay vary from embodiment-to-embodiment, such as responsive to suchfactors as the pressure within the bed, the gas volume in the bed, theflow rate of gas from the bed, etc. In FIG. 12, and the subsequentlydiscussed FIGS. 13-15, the graphs are intended to provide illustrativerepresentations of the pressure or byproduct stream flow rate as afunction of time. Because the graphs are intended for a primary purposeof illustration, the graphs are not labeled for time and insteadillustrate relative relationships between these variables.

As discussed, this change in pressure will cause many of the adsorbedgases to be desorbed from the adsorbent, and thereby withdrawn from theadsorbent bed in stream 76′. Stream 76′ also contains hydrogen gas,which was present in the bed prior to the start of the depressurizationstep. The pressure in bed 100 and/or stream 76′ will continue todecrease as the flow of gas from the bed continues. In the context offuel value, the initial flow of stream 76′ during the depressurizationstep will tend to have a different fuel value than the flow of stream76′ mid-way through the depressurization step and at the end of thedepressurization step. For example, these differences in fuel value mayreflect the relative concentrations of hydrogen gas and the respectiveones of the other gases that are present in the stream. Similarly, theflow rate of stream 76′ during these illustrative portions of thedepressurization step will also tend to vary, with the flow rate tendingto decrease during the depressurization step.

At 212, the flow of purge gas, such as in the previously discussedstream 156′ shown in FIGS. 10 and 11, is commenced to the adsorbent bed.Although not required, the volume of purge gas delivered to an adsorbentbed in a PSA assembly may be predetermined, such as to be a fixed purgevolume. The pressure of stream 156′ may vary within the scope of thepresent disclosure, but the gas is preferably at or near the pressurewithin bed 100 when the purge step begins. The flow of purge gas throughthe adsorbent portion will tend to increase the amount of desorbedgases, as the partial pressure of the desorbed gases is reduced by theflow of purge gas through the adsorbent region and then from theadsorbent bed as part of stream 76′. At 214, the flow of purge gas hasstopped. In the illustrative example shown in FIG. 12, thedepressurization, or blowdown, step is indicated as the time periodbetween times 212 and 210, while the purge step is indicated as the timeperiod between times 214 and 212. While illustrated as a distincttransition between these steps, it is not required to all embodimentsthat the depressurization of the bed be completed before the flow ofpurge gas is commenced. Instead, it is within the scope of the presentdisclosure that the pressure within the adsorbent bed may continue todecrease after the flow of purge gas has commenced.

The optimum volume of purge gas for a particular adsorbent bed may varyaccording to a variety of factors. Illustrative examples of thesefactors include one or more of the type of adsorbent being used, theconfiguration of the adsorbent bed, the size of the adsorbent bed, thepressure of the purge gas, the composition of the purge gas and/or themixed gas, the pressure of the streams (76′) exhausted from the adsorbedbed to form byproduct stream 76, etc. Therefore, an optimum purge volumethat is effective for a particular PSA assembly may not be optimum, orperhaps even effective, for a differently configured and/or sized PSAassembly.

The relative time period, or ratio of times, between thedepressurization step and the purge step may vary within the scope ofthe present disclosure, with the illustrated example shown in FIG. 12intended to illustrate but one of many suitable relationships, orratios. This ratio may be expressed as a purge-to-blowdown ratio,illustrative examples of which include 1:1 to 3:1, 1.3:1 to 2.5:1, 1.6:1to 2.3:1, 1.6:1 to 2:1, 1.6:1, 1.8:1, 2:1. 2.2:1, greater than 1.5:1,greater than 2:1, less than 2.5:1, etc. A completing designconsideration with a longer purge step, which may tend to increasedesorption and/or regeneration of the bed, is that PSA assemblies arepreferably adapted to cyclical, continuous use, with the amount of timethat a particular bed is in the depressurization and/or purge cyclepotentially affecting the amount of time that other beds may be in thesame or other steps of the PSA cycle.

The depressurization and purging of an adsorbed bed may occur within aselected time period and/or purge-to-blowdown ratio while producing avariety of flow rates and/or fuel values for stream 76′ and theresulting byproduct stream 76. For example, when the depressurizationstep begins, the adsorbent bed still contains a large amount of hydrogengas and is still at an elevated pressure. As the depressurization stepcontinues, the pressure and hydrogen gas within the bed will decrease.As the depressurization of the bed continues, the flow rate of stream76′ will decrease as the pressure within the bed decreases. Prior to thestart of the flow of purge gas to the bed, the flow of exhaust, orbyproduct, gas in stream 76′ will be relatively low, as this flow ratedecreases with the decreasing pressure in the bed. When the flow ofpurge gas commences through bed 100, the flow rate of stream 76′ willincrease, as will its fuel value when the purge gas is a combustiblegas, such as hydrogen gas, with a more particular example being purifiedhydrogen gas produced by the PSA assembly. When the purge step iscompleted, the flow of stream 76′ from that bed is stopped. When thisoccurs, the fuel stream for heating assembly 71 will be formed bystreams 76′ from one or more of the other adsorbent beds, such as whenthe beds are depressurized and/or purged during the PSA cycle, and/orfrom other sources, such as a supplemental fuel stream.

As discussed, when regenerating the adsorbent in an adsorbent bed 100 ofPSA assembly 73, purified hydrogen gas may be used as a purge stream.This flow of purge gas may induce desorption of the adsorbed gases andthereby assist in the regeneration of the adsorbent. Conventionally, theflow of purge gas to an adsorbent bed is delivered at a constant rate,typically for a fixed time period. FIG. 13 presents a graph showing theflow rate of purge gas to adsorbent bed 100 as a function of time. Thebeginning and end of the purge step are indicated in FIG. 13 at 212 and214, and correspond to the relative times and purge-to-blowdown ratiodiscussed above in connection with FIG. 12.

In dashed lines in FIG. 13, the flow rate of purge gas in a PSA cyclethat utilizes a constant, or fixed, flow of purge gas to the adsorbentbed is shown. As indicated, the gas is delivered at a constant ratethroughout the purge step, with the volume of purge gas delivered to theadsorbent bed being the product of this fixed purge rate and the timeperiod through which this purge gas is delivered to the adsorbent bed.While effective at desorbing adsorbed gases from the adsorbent in thebed, the significant increase, or pulse, in flow rate of gas in stream76′, which may be gas having high fuel value, will tend to cause asubstantial increase in the flow and/or temperature of the heatedexhaust stream from the heating assembly. This, in turn, will tend toincrease the temperature, potentially rapidly, of the structure heatedby this stream, such as the hydrogen-producing region of fuel processingsystem 64. This may result in the structure being overheated, which maydamage the structure and/or impair the operation thereof. Regardless ofany potential negative effect on the heated structure(s), the suddenincrease in the heated exhaust stream may produce exhaust gases thatexceed certain desired, or required, emissions thresholds. For example,the carbon monoxide content of the heated exhaust stream may increaseresponsive to a sudden increase in the flow of fuel to the heatingassembly.

Conversely, during equalization and/or prior to the beginning of theflow of purge gas from the bed and/or at the end of the purge step,stream 76′ may contain no flow or only low flow and the flow that existsmay have low fuel value. As a result, the heating assembly may not beable to maintain a pilot light or combustion flame without requiring aflow of fuel other than byproduct stream 76. Similarly, when the flowand/or fuel value of stream 76 is low, the heated exhaust stream may notbe able to heat the associated structure, such as hydrogen-producingregion 70, to a desired temperature or range of temperatures. Forexample, in the context of a hydrogen-producing region such as a steamreforming region that preferably operates within a selected temperaturerange to produce hydrogen gas, operating the hydrogen-producing regionat a temperature that is below (or above) the desired range will tend todecrease the amount of hydrogen gas in the mixed gas stream, therebydecreasing the conversion, or efficiency, of the hydrogen-generationassembly.

As indicated in solid lines in FIG. 13, it is within the scope of thepresent disclosure to not use a constant purge gas flow rate. Instead,the flow rate of purge gas to the adsorbent bed is varied during atleast portions of the purge step. In the illustrated example, the flowrate of purge gas begins at less than 50% of the flow rate that would berequired to deliver a determined volume of purge gas in a determinedtime period at a constant flow rate of purge gas, such as the volume andtime period represented in dashed lines. The flow rate of purge gasincreases over time from this initial rate to a maximum flow rate thatexceeds the maximum flow rate utilized in the example shown in dashedlines in which a constant flow rate of purge gas is utilized throughoutthe purge cycle. In the illustrated example, the flow rate is thenmaintained at this maximum flow rate until the flow of purge gas isstopped. At the end of the purge cycle, the flow rate of stream 76′ fromthe bed being purged has stopped; however, and within the scope of thepresent disclosure, another bed 100 of the PSA assembly is preferablyproviding a stream 76′ to maintain a substantially continuous, if notcompletely continuous, flow rate of byproduct stream 76 to the heatingassembly, with this stream preferably having a sufficient flow rateand/or heating flow to satisfy the fuel requirements of heating assembly71 and/or to maintain a continuous combustion process in the heatingassembly.

As variations of the illustrated example, the initial flow rate of purgegas may begin at 10-75% of the average flow rate that would be utilizedin a constant flow profile during the entire selected time period forthe purge step, and then increase toward at least the average purge flowrate during the first 10-60% of the purge cycle. After this, the flowrate will continue to increase beyond the average purge flow rate for arange of 10-100% of the purge step, and optionally 25-100% of the purgestep. For example, the flow rate of purge gas may increase to at least125%, 150%, 125-200%, etc. of the average flow rate that would berequired to a fixed volume of purge gas during the selected time periodfor the purge step. It is within the scope of the present disclosurethat other profiles, or ramps of the purge gas flow rate may beutilized, including profiles in which the flow rate of purge gas duringat least one portion, or subset, of the purge step increase or decreaseaccording to one or more of linear, non-linear and/or stepwise, orincremental, amounts. Illustrative examples of other profiles of purgegas flow rates through an adsorbent bed are shown in FIG. 14. As shownin FIG. 15, it is within the scope of the present disclosure that theprofile of purge gas flow rates may include a portion of reduced flow,such as during the last 5-50% of the purge step.

Any suitable method or mechanism may be utilized for regulating the flowof purge gas to the adsorbent beds. An illustrative, non-exclusiveexample is the use of a controller to selectively actuate suitableflow-regulating valves to produce the desired flow rates. As discussed,any suitable type and number of valves may be used, and it is within thescope of the present disclosure to use a different type and/orcombination of valves to regulate the flow of gas from the adsorbed bedduring the depressurization step than is used during the purge step. Itis also within the scope of the present disclosure that that the valveor valve assembly that regulates the flow of gas that will form stream76′ may be selectively used, such as responsive to control signals froma controller, to regulate the flow rate of this gas to adjust the flowrate and/or fuel value of the byproduct stream that is delivered to theheating assembly.

Preferably, the ramped, or staged, purge step as used by the PSAassembly during the PSA cycle produces a byproduct stream that, whendelivered as a fuel stream to the heating assembly, is constant, orwithin a selected range, of a determined, or selected, flow rate, suchas +/−5%, 10%, 15%, 20%, 30%, etc. of a selected flow rate. In someembodiments, the selected flow rate corresponds to a flow rate thatproduces a heated exhaust stream adapted to maintain thehydrogen-producing region of the fuel processing system at a desiredtemperature and/or within a desired temperature range, such as thosediscussed previously. Additionally or alternatively, the purge step maybe adapted to produce during the PSA cycle a flow of byproduct stream tothe heating assembly that is at a constant, or within a selected rangeof a determined, or selected, fuel value, such as +/−5%, 10%, 15%, 20%,30%, etc. of a selected fuel value. The byproduct streams preferablymaintain either or both of the above-discussed relationships to aselected flow rate and/or fuel value during at least a substantialportion, and even more preferably all of, the time period in which thePSA assembly is used to produce product hydrogen stream 42. When thebyproduct stream does not continuously meet either or both of theabove-discussed criteria, it is within the scope of the disclosure thatit may do so for at least 80%, at least 90%, at least 95%, or more ofthe cycle.

In some embodiments, the selected fuel value, as associated with theflow rate of stream 76, produces a heated exhaust stream adapted tomaintain the hydrogen-producing region of the fuel processing system ata desired temperature and/or within a desired temperature range, such asthose discussed previously. For example, the heated exhaust stream maybe adapted to maintain the hydrogen-producing region, which in someembodiments may be referred to as a reforming region, of thehydrogen-generation assembly at a relatively constant temperature, suchas a temperature in the range of 375-425° C., 400-425° C. and/or400-450° C. for methanol or similar carbon-containing feedstocks and atemperature in the range of 750-850° C., and preferably 775-825° C.,800-850° C., and/or 800-825° C. for natural gas, propane and similarcarbon-containing feedstocks.

While not required, a benefit of ramping, or incrementally increasing,the flow rate of purge gas to the adsorbent bed is that a suddenincrease in the flow rate of stream 76 to the heating assembly isprevented. Such a sudden increase may tend to produce a heated exhauststream having a concentration of at least one component, such as carbonmonoxide, that exceeds a selected threshold value, such as 50 ppm ormore. Preferably, the ramped purge step of the present disclosure isadapted to produce a heated exhaust stream that throughout the PSA cyclehas a carbon monoxide concentration of less than 50 ppm, and preferably,less than 25 ppm, less than 10 ppm, or even less than 5 ppm.

Illustrative, non-exclusive examples of implementations of the systemsand methods for ramping, staging or otherwise regulating the flow ofpurge gas to the adsorbent beds of a pressure swing adsorption assemblyinclude, but are not limited to, one or more of the followingimplementations, which may be implemented in one or more of a PSAassembly; a PSA assembly adapted to purify hydrogen gas; ahydrogen-generation assembly including a fuel processor adapted toproduce a mixed gas stream containing hydrogen gas as its majoritycomponent and other gases, and a PSA assembly adapted to produce aproduct hydrogen stream from the mixed gas stream; a fuel cell systemcontaining a fuel cell stack, a hydrogen-purifying PSA assembly and asource of hydrogen gas to be purified by the PSA assembly (with thesource optionally including a fuel processor, and in some embodiments asteam reformer); a hydrogen-generation assembly including ahydrogen-producing region adapted to produce a mixed gas streamcontaining hydrogen gas as its majority component and other gases, a PSAassembly adapted to remove impurities (including carbon monoxide) fromthe mixed gas stream (and optionally a fuel cell stack adapted toreceive at least a portion of the purified mixed gas stream), and aheating assembly adapted to combust the byproduct stream to heat atleast the hydrogen-producing region, with the hydrogen-producing regionoptionally being a steam or autothermal reforming region:

Regulating the delivery of purge gas to the adsorbent beds to maintainthe flow rate of the byproduct stream within +/−5%, 10%, 15%, 20%, 30%,etc. of a selected flow rate;

Regulating the delivery of purge gas to the adsorbent beds to maintainthe fuel value of the byproduct stream within +/−5%, 10%, 15%, 20%, 30%,etc. of a selected fuel value;

Delivering a determined volume of purge gas to an adsorbent bed during adetermined time period at a varying flow rate;

Progressively increasing the flow rate of purge gas to an adsorbent bedduring an initial percentage of the purge cycle, and optionally bymaintaining and/or decreasing the flow rate during a subsequentpercentage of the purge cycle;

Initially delivering the flow rate of purge gas to an adsorbent bed at aflow rate that is less than 75%, and optionally 50% or less of theaverage flow rate of purge gas that will be delivered during the purgestep of a PSA cycle, and thereafter increasing the flow rate of purgegas to a rate that is greater than the average flow rate of purge gasdelivered during the purge step;

After initiating the flow of the volume of purge gas, selectivelyincreasing and/or decreasing the flow rate of purge gas during the timeperiod, and optionally subsequently decreasing and/or increasing theflow rate of purge gas during the time period so that the determinedvolume of purge gas is delivered during the determined time period;

Regulating the flow rate of purge gas to the adsorbent beds of a PSAassembly to maintain the concentration of carbon monoxide in a heatedexhaust stream produced by combusting the byproduct stream from the PSAassembly below a selected threshold, such as 50 ppm, 25 ppm, 10 ppm, 5ppm, or less;

Selectively distributing a volume of purge gas to the adsorbent bedsresponsive to a predetermined flow profile having at least one portionin which the flow rate of purge gas is less than an average flow rate ofthe purge gas delivered during the purge step, and at least one portionin which the flow rate of purge gas is greater than the average flowrate of the purge gas delivered during the purge step;

Regulating the flow rate of purge gas to the adsorbent beds of a PSAassembly to maintain the concentration of at least one component of aheated exhaust stream produced by combusting the byproduct stream fromthe PSA assembly below a selected threshold value;

Selectively delivering purge gas to the adsorbent beds of a PSA assemblyin a variable-flow profile in which the flow rate of purge gas isadjusted to maintain the flow rate of gas from the PSA assembly to aheating assembly at or within a determined range of a threshold value;

Selectively delivering purge gas to the adsorbent beds of a PSA assemblyin a variable-flow profile in which the flow rate of purge gas isadjusted to maintain the fuel value and flow rate of gas from the PSAassembly to a heating assembly at or within a determined range of athreshold value;

Regulating the flow rate of purge gas to the adsorbent beds of a PSAassembly to maintain the temperature of a hydrogen-producing region of afuel processing system that is heated by a heated exhaust streamproduced by combusting the byproduct stream of the PSA assembly within atemperature range of 100° C., and preferably 50° C., or less;

Regulating the flow rate of purge gas to the adsorbent beds of a PSAassembly to provide a sufficient flow of byproduct stream to maintainthe temperature of the hydrogen-producing region within a selectedtemperature range and/or above a selected threshold value when heated bya heated exhaust stream produced by combusting the byproduct stream;

Regulating the flow rate of purge gas to the adsorbent beds of a PSAassembly to maintain a continuous flow of the byproduct stream to aheating assembly adapted to utilize the byproduct stream as a fuelstream;

Regulating the flow rate of purge gas to an adsorbent bed of a PSAassembly according to a non-linear flow profile, and optionallyaccording to a profile that includes one or more of incremental changesin flow, stepped (or step-wise) changes in flow, and non-linear changesin flow;

Any of the above systems or methods implemented with a PSA assemblyhaving a plurality of adsorbent beds adapted to receive a mixed gasstream that includes hydrogen gas as its majority component and which isproduced by a fuel processing system that includes at least onereforming region adapted to produce the mixed gas stream by steamreforming water and a carbon-containing feedstock, with at least thereforming region(s) of the fuel processing system adapted to be heatedby a heating assembly, with the PSA assembly adapted to provide at leastone fuel stream to the heating assembly, and optionally in furthercombination with a fuel cell stack adapted to receive at least a portionof the purified hydrogen gas produced by the PSA assembly;

Methods for implementing the processes of any of the above systemsand/or use of any of the above systems; and/or

A control system adapted to control the operation of a PSA assemblyand/or an associated hydrogen-generation assembly to implement any ofthe above methods or control systems.

Although discussed herein in the context of a PSA assembly for purifyinghydrogen gas, it is within the scope of the present disclosure that thePSA assemblies disclosed herein, as well as the methods of operating thesame, may be used in other applications, such as to purify other mixedgas streams in fuel cell or other systems and/or to heat structure otherthan a hydrogen-producing region of a fuel processing system.

INDUSTRIAL APPLICABILITY

The pressure swing adsorption assemblies and hydrogen-generation and/orfuel cell systems including the same are applicable in the gasgeneration and fuel cell fields, including such fields in which hydrogengas is generated, purified and/or consumed to produce an electriccurrent.

It is believed that the disclosure set forth above encompasses multipledistinct inventions with independent utility. While each of theseinventions has been disclosed in its preferred form, the specificembodiments thereof as disclosed and illustrated herein are not to beconsidered in a limiting sense as numerous variations are possible. Thesubject matter of the inventions includes all novel and non-obviouscombinations and subcombinations of the various elements, features,functions and/or properties disclosed herein. Similarly, where theclaims recite “a” or “a first” element or the equivalent thereof, suchclaims should be understood to include incorporation of one or more suchelements, neither requiring nor excluding two or more such elements.

It is believed that the following claims particularly point out certaincombinations and subcombinations that are directed to one of thedisclosed inventions and are novel and non-obvious. Inventions embodiedin other combinations and subcombinations of features, functions,elements and/or properties may be claimed through amendment of thepresent claims or presentation of new claims in this or a relatedapplication. Such amended or new claims, whether they are directed to adifferent invention or directed to the same invention, whetherdifferent, broader, narrower, or equal in scope to the original claims,are also regarded as included within the subject matter of theinventions of the present disclosure.

1. A hydrogen-generation assembly, comprising: a fuel processing systemincluding at least one hydrogen-producing region adapted to receive atleast one feed stream and to produce a mixed gas stream containinghydrogen gas and other gases therefrom; a pressure swing adsorptionassembly adapted to receive the mixed gas stream and to produce aproduct hydrogen stream containing at least substantially pure hydrogengas and having a reduced concentration of the other gases than the mixedgas stream, and to produce a byproduct stream containing at least asubstantial portion of the other gases, wherein the pressure swingadsorption assembly includes a plurality of adsorbent beds adapted toseparate the mixed gas stream into streams forming the product hydrogenstream and the byproduct stream; a heating assembly adapted to receiveand combust the byproduct stream to produce a heated exhaust streamadapted to heat at least the hydrogen-producing region; and means forregulating the flow rate of the byproduct stream to maintain thetemperature of the hydrogen-producing region within a determinedtemperature range for producing the mixed gas stream.
 2. Thehydrogen-generation assembly of claim 1, wherein the hydrogen-producingregion includes a steam reforming region adapted to produce the mixedgas stream from water and a carbon-containing feedstock.
 3. Thehydrogen-generation assembly of claim 2, wherein the carbon-containingfeedstock includes a hydrocarbon and the determined temperature range is725-825° C.
 4. The hydrogen-generation assembly of claim 25 wherein thecarbon-containing feedstock includes methanol and the determinedtemperature range is 375-450° C.
 5. The hydrogen-generation assembly ofclaim 1, wherein the means for regulating is adapted to regulate theflow rate of purge gas to the adsorbent beds of a PSA assembly tomaintain the concentration of carbon monoxide in the heated exhauststream below 50 ppm.
 6. The hydrogen-generation assembly of claim 5,wherein the means for regulating is adapted to one or more ofselectively increasing and decreasing over a time period the flow rateof the purge gas.
 7. The hydrogen-generation assembly of claim 1,wherein the means for regulating includes selectively delivering a purgestream to the adsorbent beds according to a variable-flow profile. 8.The hydrogen-generation assembly of claim 7, wherein the variable-flowprofile includes an initial portion in which the purge gas is deliveredat a flow rate that is less than an average flow rate for the purge gasthrough the adsorbent beds, and further wherein the profile includes asubsequent portion in which the purge gas is delivered at a flow ratethat is greater than the average flow rate for the purge gas through theadsorbent beds.
 9. The hydrogen-generation assembly of claim 1, whereinthe means for regulating includes means for regulating the fuel value ofthe byproduct stream.
 10. The hydrogen-generation assembly of claim 9,wherein the means for regulating includes means for maintaining the fuelvalue of the byproduct stream within a predetermined range of adetermined fuel value.
 11. The hydrogen-generation assembly of claim 1,wherein the means for regulating is adapted to ramp the flow rate ofpurge gas to the adsorbent beds.
 12. The hydrogen-generation assembly ofclaim 1, wherein the means for regulating is adapted to deliver the flowrate of purge gas at an initial flow rate of purge gas that is less than75% of an average flow rate of purge gas and thereafter increase theflow rate of purge gas.
 13. The hydrogen-generation assembly of claim 1,wherein the means for regulating includes means for selectivelydistributing a volume of purge gas to the adsorbent beds responsive to apredetermined flow profile having at least one portion in which the flowrate of purge gas is less than an average flow rate of the purge gasdelivered during the purge step, and at least one portion in which theflow rate of purge gas is greater than the average flow rate of thepurge gas delivered during the purge step.
 14. The hydrogen-generationassembly of claim 13, wherein the flow profile includes a non-linearprofile.
 15. The hydrogen-generation assembly of claim 13, wherein theflow profile includes an initial flow rate of purge gas, and at least asubsequent flow rate of purge gas that is greater than the initial flowrate.
 16. The hydrogen-generation assembly of claim 15, wherein theinitial flow rate is less than 50% of the subsequent flow rate.
 17. Thehydrogen-generation assembly of claim 15, wherein the initial flow rateis 25-75% of an average flow rate of the purge gas during a purge stageof a PSA cycle, and further wherein the initial flow rate is maintainedfor at least 10% of the purge stage.
 18. The hydrogen-generationassembly of claim 13, wherein the flow profile includes at least oneportion in which the flow rate of purge gas is decreasing with time. 19.The hydrogen-generation assembly of claim 13, wherein the means forselectively distributing is adapted to maintain the flow rate of thebyproduct stream within a predetermined range of a determined flow rate.20. The hydrogen-generation assembly of claim 19, wherein the determinedflow rate corresponds to a flow rate in which the heated exhaust streamis adapted to maintain the hydrogen-producing region within a selectedtemperature range for producing the mixed gas stream.